The software part
One of the common causes of rebooting the PC at work is the OS infected by viruses. For verification, you must update your antivirus database and scan your system. If this phenomenon started after installing any program, you should think about its removal.
There are cases when the computer restarts at boot. The reason for this is corruption of system files of the OS. In such cases, it is desirable to boot from under live CD and use the key combination Win+R to summon the Run command. Then in the input field teams to gain chkdskc: /f /r and click OK. After that you must wait until all activity is complete and close the window.
As a rule, after these transactions, the system should boot correctly. If not, you have to take care of saving the necessary information with the system drive and reinstall the OS.
Hardware
The cause of the periodic rebooting of a computer may also lie in its hardware. To find out, you need to open the PC case and to make a visual inspection for contamination, swelling, condensers and other mechanical damage. The most common cause of computer restarts is overheating of the CPU or graphics card.
Most likely, the cause of failure is clogged with dust or faulty radiator cooling fan. In this case, you must carefully remove the heatsink from the CPU and clean it from dust. It is also necessary to check for thermal paste on the CPU and apply a new one. Assembly to make in return sequence.
There are cases when you restart the computer, causes the faulty RAM. To test it you can use a number of special programs, such as Memtest86. However, the easiest way to check RAM is known-good.
The same problem may occur when you use a powerful enough power supply. Usually this happens after installation of new devices to a computer or connected peripheral devices, food that are served via the ports of the PC. It is useful to perform an autopsy of the PSU and check it for contamination.
If all the above methods do not work, to identify the reasons for the reboot is necessary to make successive replacement of all component parts. While maintaining the fault you need to check the motherboard by its replacement.